1. Field of the Invention
The present invention is directed toward an apparatus and method for controlling the draw point of a synthetic yarn.
2. Description of the Prior Art
Drawing is the process of stretching a partially oriented, or as-spun, yarn in order to develop the structure and properties required for commercial use. When compared to the undrawn yarn the desired properties include higher strength, lower elongation, higher modulus, and better recovery. Drawing is an irreversible elongation of the filaments of the yarn to as much as six times the original length for nylon, and even more for other yarns.
In the usual commercial environment drawing is usually accomplished in a continuous manner. Two rolls are required for continuous drawing, with the second (upstream) roll running at a faster speed than the first (downstream) roll. The "draw ratio" is the ratio between the surface speed of the second roll divided by the surface speed of the first roll.
The draw point is the location at which the necking, or narrowing, of the yarn occurs. Upstream of the draw point the undrawn yarn travels at a first velocity and exhibits a first predetermined cross-sectional area. However, the drawn yarn downstream of the draw point exhibits a second, narrower, cross-sectional area and travels at a commensurately higher velocity.
It is known that both operability of the yarn manufacturing processing and the uniformity of the drawn yarn can be improved by localizing the draw point. One common method of doing this is to locally heat the yarn to lower its yield point. This may be accomplished by heating one or more of the feed rolls that are upstream of the draw point. In addition, it is also a common practice to heat the downstream roll so that the drawn yarn may be bulked.
In the ideal case, it is desirable for all of the draw to occur in the clearance space between the two rolls. This condition is referred to as "space draw". Care must be exercised to insure that drawing of the yarn occurs away from the heated surface of either the upstream and/or the downstream roll. If drawing of the yarn is attempted on a heated surface, the possibility of yarn breakage is increased.
In view of the foregoing it is believed to be advantageous to provide an apparatus to control the location of the draw point so that the draw point is caused to occur in space intermediate the upstream and downstream rolls.